The present invention is directed to a composite film structure having superior barrier properties to atmospheric gases as measured by low oxygen transmission together with low thermal transfer to the edges of the film structure. The composite is particularly suitable for commercial fabrication of vacuum insulation panels for refrigerators, ovens and shipping containers.
In the construction of vacuum insulating panels it is well known that a high vacuum significantly improves the insulating properties and the use of a metal foil provides a superior barrier to transmission of atmospheric gases. The use of a metal foil in an insulating composite film, however, inherently provides a tradeoff in properties such as higher weight, thicker film, and undesirable heat conduction along the surface of the film. Therefore, composite film constructions are desired which replace the metal foil in a film composite and yet maintain acceptable levels of impermeability to atmospheric gases. In addition, replacement of a metal foil in a composite film structure with a more flexible element resistant to cracking advantageously facilitates reliable automated production and may provide an equal or even improved impermeability to atmospheric gases.
The present invention is an improvement over Revell GB 2210899, assigned to Bowater Packaging Limited, which discloses metallized plastic film composites having low permeability to gases, moisture and light comprising a thermoplastic film metallized on at least one surface, overcoated on the metal surface with a substantially continuous thin plastic coating less than 10 microns in thickness and remetallized on the coated surface. The preferred metal is vacuum deposited aluminum and plastic coatings include thermoplastic lacquers or inks based on resins such as polyester, nitrocellulose, acrylic, vinyl or polyvinylidene chloride with solvent-based polyester lacquers being preferred. However, the teachings of Revell do not provide a composite film having barrier properties comparable to the composite films of the present invention.
Revell in U.S. Pat. No. 5,021,298, also assigned to Bowater Packaging Limited, further discloses barrier improvements using single vacuum metallized layers applied over smooth thin polymeric layers without the need for any second metallized layer.
The teaching of both of the Revell patents is clearly directed to the improved barrier properties obtained using vacuum metallized layers deposited on smooth surfaced polymeric substrates as alternatives to obtaining improved barrier properties using multiple vacuum deposited metal layers.
The following prior art is generally directed to insulating film composites providing barrier properties to air, particularly to oxygen.
Yoneno et al. U.S. Pat. No. 4,594,279, assigned to Matsushita Electric, discloses a vacuum-filled, pliable, heat insulating container containing flaky pearlite particles made from a single layer or a laminated film which can contain an aluminum deposited film or an aluminum foil. The patent discloses a thermal conductivity lower than 0.01 Kcal/mh.degree. C.
Moretti U.S. Pat. No. 4,662,521, assigned to U.S. Phillips, discloses a thermal insulation bag composed of a multilayer laminate of polyethylene, aluminum foil and polyester.
Engelsberger U.S. Pat. No. 4,937,113, assigned to Helio Folio, discloses a multilayer film for vacuum packaging containing a support film which includes a plurality of layers including an aluminum film having a thickness of 7 microns.
Carr et al. U.S. Pat. No. 5,018,328, assigned to Whirlpool Corporation, discloses a vacuum insulation panel construction having a plurality of layers of gas impermeable material in conjunction with a metal foil for providing a thermal break. The foil is used on one side of the panel construction and the gas impermeable material is used on the other side to create the thermal break. However, this construction greatly reduces the life of the vacuum panel because the average barrier properties of the assembly include both the average barrier level of the foil and the reduced barrier level of the non-foil material.
Kirby U.S. Pat. No. 5,091,233, also assigned to Whirlpool Corporation, discloses vacuum insulation panels which contain a plurality of layers including a barrier film composed of a layer of metallized or nonmetallized plastic and a layer of metal foil. The patent points out the importance of thermal conduction along the surface of the panels. Inherently, a thermal break is formed as a spacing or gap in the metal foil used in the film composite. This thermal break prevents conduction of heat around the exterior of the vacuum panel from hot to cold side and compromises the total barrier level of the assembly.
Walles U.S. Pat. No. 3,993,811, assigned to Dow Chemical, discloses a thermal insulating panel having a double wall construction composed of a structural plastic material with improved thermal insulative properties due to reduced permeance of atmospheric gases into an evacuated center. The panel material is a thick structural plastic from 40 to 300 mils thick, which is fi metallized (0.0001 to 0.5 mil) and then overcoated with a barrier polymer by latex coating (0.001 to 2.5 mil). The preferred barrier polymer coating is polyvinylidene chloride copolymer. A surprising increase in barrier is reported by overcoating a metal with a barrier polymer as compared with a metallized structure plus a barrier polymer layer. The lowest oxygen transmission rate disclosed for the structure is 0.013 cc/square meter-day-atm.
Accordingly, there is still a need for a film composite which has superior properties to passage of oxygen together with the ability to act as an insulator.